STRUCT DETERMINATION OF UNIFORMLY LABELED MOLECULES BY SSNMR DIPOLAR RECOUPLING

点击翻译按钮获取中文摘要

We present a variety of structural constraints derived from dipolar recoupling in uniformly- "C, "N-labeled small molecules, including the chernotactic peptide formyl-Met-Leu-Phe and the macrolide antibiotic erythromycin A. Extensions of HCCH and HNCH torsion angle methods exploit the newly developed T-MREV sequence (Hohwy and co-workers) and employ two chemical shift dimensions to provide simultaneous constraints of several projection angles. For optimal efficiency, homonuclear polarization transfer is performed with the class of y-encoded double-quantum sequences (C7 and its compensated analogs), and heteronuclear transfer by adiabatic cross-polarization. Our approach is compared with previous methods, such as those that rely upon multiple ic pulse 'H-X recoupling or local field evolution on the double-quantum coherence. In the former case, substantial artifacts are introduced from the n pulses; in the latter, constraints are limited to directly bonded neighbors. Measuremen ts over multiple bonds provide substantial advantages versus single bond measurements for structure determination, because redundant constraints of individual torsion angles from separate 'HXX2H projection angles reduce propagation of error. The complications of CH2 and CH3 groups, relaxation, and numerous weaker couplings are modeled with highly efficient multi-spin numerical simulations. In addition to the 'HX based constraints, we demonstrate application of the 15N 13C 13C 15N 14 measurement, with the requisite resolution of chemical shifts in the double quantum 13C_13 C dimension. For distance measurements in uniformly labeled systems, we explore two possibilities. For heteronuclear distances, variations of the REDOR experiment and sele * ctive CP approaches are compared. For homonuclear spin pairs, we consider the extent to which spin pairs with naturally advantageous chemical shift differences can be exploited to extract intra- and inter-residue distances. For example, rotational resonance tickling (R2T) is used to recouple CO-CH3 pairs and selective laboratory and rotating frame recoupling experiments are used for CO-CO or CH3-CH3pairs. In all experiments discussed, the efficiency of polarization transfer is excellent (>50%) and internal controls are present. Therefore, these methods may be extended to larger biomolecules, assuming sufficient resolution and sensitivity exist.